The present invention relates to static power converters for induction heating, and more specifically to a gate control circuit for repititiously gating, a silicon-controlled rectifier incorporated in an inverter, at ultrasonic frequency.
When a forward voltage is applied rapidly to a silicon-controlled rectifier, switching can occur by a mechanism commonly called dv/dt triggering. Basically, this type of switching occurs as the depletion region of its center pn junction adjusts to accommodate the increasing voltage. As the depletion width of the center junction increases, holes and electrons are removed from the adjacent regions of the junction. For a slow increase in voltage, the resulting flow of holes and electrons does not constitute a significant current. If dv/dt is large, however, the rate of charge removal from each side of the center junction can cause the current to increase significantly. If the silicon controlled rectifier has a small dv/dt rating, the device will be turned on by the reapplied forward voltage immediately after the forward commutating current falls to zero before the device is normally turned on by a positive gating signal.
To turn off the silicon-controlled rectifier in a minimum time if it is necessary to apply a reverse voltage. When this reverse voltage is applied, the holes and electrons in the vicinity of the two end junctions will diffuse to these junctions and result in a reverse current in the external circuit. After the holes and electrons in the vicinity of the two end junctions have been removed, the reverse current will cease and the end junctions will assume a blocking state. The reverse voltage across the device will then increase to a value determined by the external circuit. Recovery of the SCR is not complete, however, since a high concentration of holes and electrons would still exist in the vicinity of the center junction. This concentration decreases by the process of recombination.
When an SCR is used as a switching element in high frequency applications such as inverters, the dv/dt capability and its turn off time become an important characteristic. Turn off time is defined as the time interval required for the device to regain its forward blocking state after forward conduction.
In order to prevent the silicon-controlled rectifier from becoming conductive immediately after the forward conduction due to the rapid rise in the reapplied forward anode voltage, a prior art induction heating apparatus, such as disclosed in U.S. Pat. Nos. 3,710,062 and 3,821,509, employs an RC snubber circuit to prevent the premature switching action. However, the use of the RC snubber circuit entails loss of usable energy for induction heating.